The invention relates to Superpave Asphalt Mix design.
The design of asphalt paving mixes mainly involves selecting and proportioning materials to obtain the desired properties in the finished construction. The overall objective for the design of asphalt paving mixes is to determine an economical blend of binder and gradation of aggregates, within the limits of the project specifications, and an asphalt paving mixture that yields a mix having: sufficient asphalt to ensure a durable pavement; sufficient mix stability to satisfy the demands of traffic without distortion or displacement; sufficient voids in the total compacted mix to allow for a slight amount of additional compaction under traffic loading without flushing, bleeding, and loss of stability, yet low enough to minimize the intrusion of harmful air and moisture; and sufficient workability to permit efficient placement of the mix without segregation.
From October 1987 through March 1993, a DOT program known as Strategic Highway Research Program (SHRP) conducted research effort to develop new ways to specify, test, and design asphalt materials. The final product of the SHRP asphalt research program is a system referred to as Superpave, which stands for Superior Performing Asphalt Pavements. It represents an improved system for specifying the components of asphalt concrete, asphalt mixture design and analysis, and asphalt pavement performance prediction.
Superpave mix design is a structured approach consisting of the following four steps:
selection of materials,
selection of design aggregate structure,
selection of design asphalt binder content, and
evaluation of moisture susceptibility.
The selection of materials is accomplished by first selecting a Performance Grade asphalt binder for the project climate and traffic conditions. Superpave binders are designated with a high and low temperature grade, such as PG 64–22. For this binder, “64” is the high temperature grade and is the 7-day maximum pavement design temperature in degrees centigrade for the project. The low temperature grade, “−22,” is the minimum pavement design temperature in degrees centigrade. Both high and low temperature grades are established in 6-degree increments. Thus, the binder grade is an indication of the project-specific temperature extremes for which the asphalt mixture is being designed.
In addition to climate, traffic speed and traffic level may also influence Superpave binder selection. A project with slow moving or stationary traffic would require a binder with one or two higher temperature grades than would otherwise be selected on the basis of climate alone. Projects with very high traffic levels in excess of 30 million 80 kN equivalent single axle loads would also require an increase in high temperature binder grade.